In a conventional, long-haul wavelength division multiplexing (WDM) submarine transmission system, a submarine section including a submarine repeater is designed optimally such that required transmission characteristics are satisfied with the maximum number of wavelengths of the system. In general, however, the system comes into operation initially with a smaller number of wavelengths (for example, from several percent to about 10 percent) to optimize the initial construction costs.
In this case, the optical output from the submarine repeater is controlled to be constant assuming the maximum number of the wavelengths. Thus, if the initial number of the wavelengths is smaller than the maximum number, the power of each signal channel increases and the transmission quality deteriorates due to the nonlinear effect in the transmission path. A technology is known that combines and transmits dummy light (DL) in an unused band to prevent the deterioration and ensure transmission quality. The optimization of transmission characteristics for each signal is also demanded of a long-haul transmission, resulting in demand for technology that adjusts the level of signal light by adjusting the level of the dummy light.
In a long-haul transmission system, the cumulative effect of characteristics of fibers and/or submarine devices of multiple stages results in transmitted signals being affected by cumulative dispersion and/or the nonlinearity of the fibers. Thus, the waveform is significantly deteriorated, and the level of the transmitted signal on each line needs to be optimized. In particular, a system that includes an optical add drop multiplexer (OADM) requires a technique to optimize upstream and downstream trunk lines and, upstream and downstream branch lines separately, while minimizing the system costs and ensuring the security of the signal.
An OADM configured by combining a fiber grating and a circulator is conventionally known (see, for example, Japanese Laid-Open Patent Publication Nos. H10-150433 and H11-275007). In this configuration, the level of an add signal on a branch line is adjusted by inserting dummy light from a branch station when the add signal is combined to a signal on a trunk line.
However, it is difficult in the conventional technology to adjust the power of dummy light on a line that transmits signal light from a communication device, in which a complicated control mechanism and/or a light source (such as a submarine branching unit) is difficult to dispose, separately from the power of dummy light on the other lines. Thus, the power of the signal light amplified by a relay deviates from the appropriate range depending on the number of wavelengths of the signal light and the communication quality deteriorates.
For example, it is difficult to adjust the power of dummy light on a drop line from the submarine branching unit, separately from the power of dummy light on a trunk (through) line of and/or an add line to the submarine branching unit.